Magnetic Resonance Imaging (MRI) is one of the most important modern medical imaging modalities. It has far less risk of side effects than most other imaging modalities such as radioscopy with x-rays or computed tomography because patient and medical personal are not subjected to ionizing radiation exposure in the procedure. The use of MRI has grown very fast. Every year, more than 30 million MRI scans are performed in the United States; more than 60 million MRI scans are performed worldwide. Doctors often recommend MRI for the diagnoses of various diseases, such as tumors, strokes, heart problems, and spine disease. A high-quality scan is important for maximizing diagnostic sensitivity and making the right diagnosis. Generally, a high quality image requires high signal to noise ratio (SNR), high contrast between normal and pathological tissues, low levels of artifact, and reasonable and acceptable spatial-temporal resolution.
In order to obtain a detectable MR signal, the object examined is positioned in a homogeneous static magnetic field so that the object's nuclear spins generate net magnetization oriented along the static magnetic field. The net magnetization is rotated away from the static magnetic field using a radio frequency (RF) excitation field with the same frequency as the Larmor frequency of the nucleus. The rotated angle is determined by the field strength of the RF excitation pulse and its duration. In the end of the RF excitation pulse, the nuclei, in relaxing to their normal spin conditions, generate a decaying signal (the “MR signal”) at the same radio frequency as that used for excitation. The MR signal is picked up by a receive coil, amplified and processed. The acquired measurements are digitized and stored as complex numerical values in a “k-space” matrix. An associated MR image can be reconstructed from the k-space data, for example, by an inverse 2D or 3D fast Fourier transform (FFT) from raw data, which are collected in the spatial frequency domain (the “k-space”).